1. Field of the Invention
The present invention relates to an automatic transmission for an automobile and, more particularly, to an automatic transmission including a gear train composed of a plurality of planetary gear sets.
2. Description of the Prior Art
As is well known in the prior art, the gear trains of an automatic transmission are constructed of a plurality of planetary gear sets of single-pinion type, double-pinion type or Ravignawx type. For the larger number of the planetary gear sets to be used, the number of settable gears is accordingly increased the more to improve the power performance of the vehicle and the accelerability at the start, whereas the structure is accordingly complicated to raise the weight and the production cost. For the smaller number of the planetary gear sets to be used, on the contrary, the structure is simplified, although the number of settable gears is decreased, so that the lighter product can be obtained at the lower cost. In the prior art, therefore, various endeavors have been made to satisfy the contradictory requirements for the increase in the number of the gears and for the simplification of the structure. In recent years, there has been proposed a gear train which can set five forward gears and one reverse gear or more gears by using three planetary gear sets.
One example of the proposals is described in U.S. Pat. No. 4,233,861. The gear train is equipped with one of Ravignawx type planetary gear set and one of single-pinion type planetary gear set. The Ravignawx type planetary gear set has its one pinion gear meshing with a first large-diameter sun gear and a ring gear and its other pinion gear meshing with a second small-diameter sun gear and the former pinion gear, and these pinion gears are carried by a planetary carrier. On the other hand, the ring gear of the Ravignawx type planetary gear set is connected to the ring gear of the single-pinion type planetary gear set. The first sun gear is connected through a first clutch to an input shaft, and the carrier of the Ravignawx type planetary gear set is connected through a second clutch and a one-way clutch to the sun gear of the single-pinion type planetary gear set. There are further provided a third clutch for connecting the second sun gear and the input shaft, and a fourth clutch for connecting the carrier of the Ravignawx type planetary gear set and another input shaft. There are further provided a first brake for braking the rotations of the first sun gear, a second brake for braking the rotations of the sun gear of the single-pinion type planetary gear set, and a third brake for braking the rotations of the carrier of the Ravignawx type planetary gear set.
In the automatic transmission described in this U.S. Pat. No. 4,233,861, the aforementioned first, third and fourth cluthes are interposed between the Ravignawx type planetary gear set and the input shaft. As a result, the connecting members for connecting those clutches and the aforementioned first and second sun gears or carriers have to be elongated to raise a disadvantage that the weight is augmented. Moreover, the connecting members have to take a complicated tripple-tube structure in which three tubes are concentrically arranged. Since, furthermore, either an input shaft or a rotating member for connecting the input shaft has to be disposed around the fourth clutch, the oil passage for feeding the oil pressure to a hydraulic servo mechanism for engageing the fourth clutch or the third clutch may be complicated.
Another automatic transmission having three planetary gear sets and capable of setting at least five forward and one reverse speeds is described in U.S. Pat. No. 4,660,439. This automatic transmission includes: a first clutch arranged at the side of an input shaft for connecting the ring gear of a first planetary gear set and the input shaft; a second clutch for connecting the sun gear of the first planetary gear set and the input shaft; a third clutch arranged adjacent to the first planetary gear set for connecting the sun gear of a second planetary gear set and the second clutch; and a fourth clutch for connecting the carrier of the second planetary gear set and the second clutch. Since, however, the fourth clutch is interposed between the second planetary gear set and the third planetary gear set and since the ring gear of the second planetary gear set and the ring gear of the third planetary gear set are connected, a member for connecting those ring gears must be positioned around the fourth clutch. This positioning restricts the size of the external diameter of the fourth clutch so that the fourth clutch has to be axially elongated to retain a necessary transmission torque capacity. As a result, there arises a disadvantage that the whole length of the automatic transmission is increased. In addition to the fact that the rotating member is arranged around the fourth clutch, a quill shaft for connecting the aforementioned two ring gears and the carrier of the first planetary gear set is disposed inside of the fourth clutch. As a result, the structure of an oil passage for feeding the oil pressure to a hydraulic servo mechanism for engaging the fourth clutch may be complicated.